Cavity evacuation in plastic injection molds

Bibin, I have used and endorsed cavity evacuation for many years. During the '80s the trend in molding and product design was to "Thinwall" applications which drove a whole new review of processing. Anyone who has experienced "High Speed", "High Pressure" Injection using "Accumulated" Presses can identify with the typical "Dieseling" or "Burn Spots" (on parts and on mold surfaces) that occur when trapped gas within the mold cavity is compressed to a point of ignition. Since we use resins that are primarily petrochemicals, it is obvious that the gas produced when the resin is heated is combustable----and this gas fills the cavity in front of the actual flow of molten resin and is driven into various corners based on the flow pattern during fill. We had found that in many high speed applications normal Parting Line venting and/or vented Ejector Pins DID NOT have enough evacuation rate to overcome the "Dieseling", and I was introduced to a Vacuum System named "Mold-Vac". This system is very easy to initiate in any mold design and is portable within a molding facility whereas is can be plugged in to whichever tool is running. The big advantage is that during the injection cycle you have an evacuated cavity----thus avoiding ant gas pressure build up and potential Dieseling. Another secondary effect is that the Injection and/or Packing Pressures are typically reduced because you are not fighting the resistance of compressed gas in the cavity. Another point I can attest to is that when a High Speed, High Pressure, Thinwall application runs without the Vacuum System, you should expect errosion in the steel at each point of where the trapped gas results in dieseling-----as this ignition happens during every molding cycle. I hope my comments help.

06-18-2014 08:11 下午

Top #3

Bibin Roy

Thanks for the tips...Can you suggest a best supplier for this?

06-19-2014 02:42 上午

Top #4

Ken Adams

You can now get small vacuum units which can be attached to the mould from mould die base and accessory suppliers which can often be a convenient way to apply the method